For several decades, electronic devices have been implanted in humans and animals. Such devices are generally used to monitor or regulate the functions of body organs and the like. For example, a pacemaker monitors and regulates heart rate, delivering electrical impulses as required to maintain a satisfactory heart beat. Such implantable devices are powered by batteries which have a finite capacity, and which also present a limitation in terms of available peak power. As a result, there is a significant need to minimize the energy expended during operation of the implantable device, and to reduce the peak power required for special situations such as communications with external devices.
In recent years, implantable electronic device technology has rapidly advanced. Sizes and weights have decreased, while functionality has increased. These advances have created a corresponding demand for two-way communication or telemetry between the implanted electronic device and an external device, generally known as a programmer. In a pacemaker system, for example, a programmer downloads data to an implanted pacemaker, such as operating parameters. Likewise, data may flow in the opposite direction, that is from the implanted device to the programmer for analysis. In fact, modern pacemakers are capable of storing significant amounts of data about the patient (e.g., average heart rate) and the pacemaker (e.g. battery voltage) which may need to be frequently telemetered or transmitted to the programmer for evaluation by the physician.
Modern telemetry systems, however, are subject to competing requirements. They must transmit large amounts of data in a reasonable rapid manner. They must also transmit such data while using as little current from the battery as possible. Finally, they must meet these requirements while typically housed within a metallic hermetic canister.
Most modern implantable devices feature a metallic hermetic canister in order to withstand prolonged exposure to the harsh in vivo embodiment. Numerous factors, including human immune response, chemical reactivity, temperature, mechanical stresses, and the like, contribute to the harshness of this environment on implanted objects. As a result, every component of an implantable device must be biostable and biocompatible, or else surrounded or encased in a container or coating which is biostable and biocompatible.
A cardiac pacemaker, for example, typically features a hermetically sealed metallic housing, such as titanium. In particular, titanium has been found to be one of the few acceptable metals from which the hermetic canister may be made. Several other metals may also be acceptable, such as gold or platinum, but these are not commercially acceptable due to their cost. Other metals are generally unsuitable for implant in the body due to their tendency to corrode. Moreover, most other materials, such as many plastics and other synthetic materials, which appear to be suitably resistant to the environment in the short term, nonetheless permit seepage of body fluids over the long term.
One advantage of using a metal for the hermetic canister is that the canister itself is conductive. This conductivity may be exploited, for example, by utilizing the canister itself as an electrode.
The conductivity of the implanted device enclosure, however, also has at least two significant disadvantages in regards to the telemetry system. First, the metal enclosure attenuates the electromagnetic field strength of signals transmitted to and from the implantable device. During uplink or communication from the implanted device, this attenuation may be overcome by increasing the strength of the transmitted signal. This increased signal strength, however, increases the current drain on the battery thereby diminishing the device longevity. Thus, in most cases, it is necessary to place the external programming device as close as possible to be implantable medical device so the signal strength may be thus be kept as low as possible and the battery not unnecessarily depleted. Second, the metallic housing also has a limiting effect upon the rate at which data may be transmitted between the implantable device and the external program.
Of course, a device having its antenna disposed outside the metallic hermetic canister would not suffer from these drawbacks. Such an antenna, however, because it must be either biocompatible or encased within a biocompatible material and must as well as be of a suitable configuration and have suitable performance, has not been developed to date. Thus, there is a long-felt need for a more efficient telemetry system for use with implantable devices, and specifically for an improved antenna which is both biocompatible and biostable so that it may be positioned outside of the metallic hermetic canister.